1. Field of the Invention
This invention relates to a controlled free radical oligomerization of tetrafluoroethylene and a means of introducing carboxylic acid functionality at the ends of each chain.
2. Description of the Prior Art
Synthetic routes to perfluorodicarboxylic acids have commonly involved the oxidative cleavage of carbon-carbon double bonds such as those present in perfluorocycloalkenes or perfluorodiolefins. The free radical polymerization of tetrafluoroethylene to give high molecular weight insoluble and somewhat intractable polymer is known. The use of aqueous peroxydisulfate as an initiator has been shown to produce high molecular weight insoluble polymers is also known. However, the established use of free radical initiator concentrations for tetrafluoroethylene is substantially below 0.5% by weight of the reaction medium, and preferably below 0.1% by weight.
The paper by K. L. Berry and J. H. Peterson, J. Am. Chem. Soc., 73, 5195 (1951) describes the free radical polymerization of tetrafluoroethylene using aqueous peroxydisulfate in concentrations of about 0.05% by weight, and ferrous iron concentrations of about 2 parts per million. However, this polymerization of tetrafluoroethylene produced a high molecular weight insoluble and somewhat intractable polymer with number average molecular weight of about 142,000 to about 534,000.
The paper by M. I. Bro and C. A. Sperati, J. Polymer Sci., 38, 289 (1959), also describes the use of aqueous peroxydisulfate initiator for the polymerization of tetrafluoroethylene which produced only high molecular weight polymer (350,000 to 2,150,000 g/m). Concentrations of the peroxydisulfate were low, and ranged from about 0.2 to about 0.4% by weight. Furthermore, only a small amount of the peroxydisulfate was consumed because the polymerization was performed in the absence of a promoter.
Syntheses of .alpha.,.omega.-perfluorodicarboxylic acids are known. The Kirk-Othmer Encyclopedia of Chemical Technology, (2nd Ed., Wiley: New York, 1970) Vol. 9, pg. 774, describes the individual syntheses of HOOC(CF.sub.2).sub.n COOH wherein n=1, 2, 3, 4. The paper by R. A. Mitsch, P. H. Ogden, and A. H. Stoskopf, J. Org. Chem., 35, 2816 (1970) describes the synthesis of .alpha.,.omega.-perfluorodicarboxylic acids wherein n=6, 9, and 12. Other papers describe the synthesis of .alpha., .omega.-perfluorodicarboxylic acids wherein n=4, 6, 8, 12 (I. L. Knunyants, Chih-Yuan Li, and V. V. Shokina (Chem. Abs. 56, 302 (1962)); and wherein n=3, 4 (E. T. McBee, P. A. Wiseman, and D. D. Bachmann, Ind. Eng. Chem., 39, 415 (1947)). However, none of the above references describe the use of relatively high, above 0.5% by weight, free radical initiator concentrations for the synthesis of .alpha.,.omega.-perfluorodicarboxylic acids and the mixtures formed thereby, by the methods of the present invention.
It is therefore an object of the present invention to provide a synthesis of .alpha.,.omega.-perfluorodicarboxylic acids via a controlled free radical oligomerization reaction.
It is a further object of this invention to produce mixtures of said .alpha.,.omega.-perfluorodicarboxylic acids of both even- and odd-numbered chain lengths.
It is a further object of this invention to use high concentrations of free radical initiators in order to control oligomerization.